A Manual Of Photography

82 HISTORY OF PHOTOGRAPHY. The excess of the nitrate of silver makes the paper blacker where the light did not act on it, and this penetrates the paper; whereas the darkening produced by the light is confined to the surface. The maximum intensity of the spectrum on the paper, when a prism of crown glass is used, lies between the indigo and blue ray. The difference of effect of a strong and weak light is beautifully shown in the action of the spectrum: that part of the paper which is exposed to the indigo ray is coloured a red- dish brown, and this is gradually darkened towards either ex- tremity, until it becomes a deep black. " I have not had many opportunities of experimenting with the catalysotype, but it certainly promises to repay the trouble of further investigation. The simplicity of the process, and the sensibility of the paper, should cause it to be extensively used. It has all the beauty and quickness of the calotype, without its trouble, and very little of its uncertainty; and, if the more fre- quent use of it by me, as compared with other processes, does not make me exaggerate its facility of operation, I think it is likely to be practised successfully by the most ordinary experimenters." Dr. Woods subsequently made the following addition:— “ Since the preceding paper was written, I have been ex- perimenting with the catalysotype, and one day having had many failures, which was before quite unusual with me, I am induced to mention the cause of them, for the benefit of sub- sequent experimenters. The paper I used was very stiff and highly glazed, so that the solution first applied was not easily imbibed. The blotting paper was very dry and bibulous. When using the latter, I removed nearly all the solution of iron from the first, and, of course, did not obtain the desired result. “While varying the process in endeavouring to find out the cause just mentioned, I discovered that the following proportions »ave very fine negative pictures, from which good positive ones were obtained:—Take of syrup of ioduret of iron, distilled water, each two drachms ; tincture of iodine, ten to twelve drops : mix. Tirst brush this over the paper, and, after a few minutes, having dried it with the blotting paper, wash it over in the dark (before exposure in the camera) with the following solution, by means of a camel-hair pencil:—Take of nitrate of silver one drachm; pure water one ounce : mix. This gives a darker picture than the original preparation, and consequently, one better adapted for obtaining positive ones : it also requires no previous steeping in an acid solution. To fix the picture let it be washed first in water, then allowed to remain for a few minutes in a solution of hydriodate of potassa (five grains to the ounce of water) and washed in water again. The paper I use is the common unglazed